1. Field of the Invention
The present invention relates to an interferometer that contains waveguides and a 3xc3x973 directional coupler.
2. Background Information
Interferometers are often employed in metrology, remote sensing, and process control applications in which the property or process parameter of interest is encoded as a phase shift between a sample and reference optical beam.
For example, in the disk drive industry, determining the microscopic topology of a disk surface at various stages during production is becoming an increasingly important factor in estimating the likelihood of producing a favorable quality disk. Cost pressures are increasingly forcing manufacturers to weed out unviable disks at earlier stages of production. Given the very high density of data stored on such disks, topographic profiles with heights ranging from less than 1 nm to tens of microns must be monitored at a lateral resolution on the order of 1 micron. Given the throughput of disks required for cost effective use of capital, a disk should be inspected in a period on the order of 10 seconds to 1 minute. This means that the inspection apparatus would need an effective data collection rate on the order of 100 to 600 MHz to inspect one side of a 95 mm diameter disk. The technical challenge is then to produce a very sensitive, high speed interferometric system that has a wide dynamic range. The system should also be accurate, compact, reliable, and cost effective.
Fundamentally, interferometers are devices which convert the phase difference between two input waves into intensity variations on one or more output waves which carry information about the phase difference between the input waves. The interferometer outputs are superpositions of portions of the two input waves. The amount of each input delivered to each output, and the phase shift imparted during delivery determines the characteristics of the interferometer.
For example, in the canonical interferometer employing a beamsplitter as shown in FIG. 1, two input beams with electric field magnitudes Ea and Eb are each split into two components of equal magnitude by the beamsplitter. However because of the reflections at the interface within the beamsplitter, the phase of the component which is reflected in the beamsplitter is shifted 90 degrees with respect to the phase of the component which is transmitted. The two key properties of this interferometer that determine its operational characteristics are i) that the inputs are split into equal magnitude components, and ii) that the phase shift imparted to one of those components is 90 degrees.
The consequences of this interferometer""s properties may be seen by inspecting the results of the math describing these properties:
input beam 1: Ea ei((w)t) 
input beam 2: Eb ei(wt+Ø) 
ouput beam 1: xc2xd (Ea ei(wt)+Eb ei(wt+Øxe2x88x92xcfx80/2))
ouput beam 2: xc2xd (Ea ei(wtxe2x88x92xcfx80/2)+Eb ei(wt+Ø))
Here, w is the optical frequency of the light beams, Ø is the phase relation between input beam 1 and 2, and xcfx80/2 represents the 90 degree phase shift incurred by the reflection within the beamsplitter.
The intensities of the two output beams are proportional to the magnitude squared of their component electric fields, so the measurable intensities of the two beams are:
output beam 1:=xc2xd ( Ea2+Eb2+2 Ea Eb sin(Ø))
output beam 2:=xc2xd ( Ea2+Eb2xe2x88x922 Ea Eb sin(Ø))
These equations are the canonical equations describing interference between two waves, and are illustrated in FIG. 2 where Ea=Eb. The intensity of each measured output beam is sinusoidally modulated from minimum to maximum as a function of the relative phase difference Ø between the two input beams. Also, the second output beam is modulated in exact opposition with respect to beam 1.
Together, the combined intensity of the two beams conserves the combined intensity of the input beams, but aside from this, the phase information in the second output beam is entirely complementary to the phase information carried in the first beam. In this sense, the second beam provides only redundant information about the phase Ø. The fact that the second output is modulated in exact opposition to the first is a direct consequence of the property of the beamsplitter that imparted a 90 degree phase shift on the reflected beam components. It is not a fundamental aspect of interferometry or of a generalized interferometric apparatus.
As simple as this canonical interferometer is, it has a number of undesirable characteristics, the foremost being that the output intensity, which carries the phase information between the input beams, is bounded and periodic while the input phase difference is not. Consequently, as the intensities of both output beams reach their respective minima or maxima, the sensitivity of the interferometer to changes in the input phase-difference drops to zero. Said another way, the sensitivity of the interferometer is proportional to the slope of the intensity vs. phase-difference relation. Since the slope goes to zero at the maxima and minima, the sensitivity drops to zero there. Not only is it undesirable to have variable sensitivity, but the complete null in sensitivity leads to an inability to unambiguously track phase-difference excursions beyond xc2x1xcfx80/2 from the point of maximum sensitivity. A reversal in the progress of a phase-difference occurring within the blind region of the interferometer could not be discerned from a continuation of the phase-difference into the next order.
The limitations of this canonical xe2x80x9chomodynexe2x80x9d interferometer are widely recognized, and a number of methods have been developed to bypass these limitations. Chief among the methods used is the Doppler, or heterodyne, interferometer. In this form of an interferometric device, the same form of beamcombining described above may be used to combine the two test beams, however one of the beams to be interfered has had its frequency shifted with respect to the other beam (for example by the use of an acoustic-optic modulator or by using a dual-frequency laser). Because of this, when the two input beams are interfered, the optical frequency does not completely drop out of the equations describing the intensity-modulated output. Instead, the output intensity is modulated at the frequency shift employed (typically 10-50 MHz). Stated differently, the output appears to register a constantly increasing phase difference between the two input beams. In this way, an externally induced phase shift between the beams is detected as a momentary change in the rate of phase advance of the output intensities.
The advantages of Doppler interferometers include an ability to unambiguously detect phase shifts in either direction as well as shifts over ranges exceeding one interferometric order. Disadvantages of Doppler interferometers include the increased complexity since a dual frequency source must be used, and the additional RF electronic stages that are needed to track the phase of the detected signal. Furthermore, the rate of phase shift that can be tracked with a Doppler interferometer is limited by the frequency shift employed and the degree of filtering used in the phase detection electronics. A tradeoff between tracking speed and minimum detectable motion is required because of this. It is therefore impossible to detect very slow motions (e.g. small displacements with short periods) with high bandwidth. Likewise, very high speed motions can only be detected with limited accuracy. In no case should the rate of phase shift exceed the bias phase shift rate set by the frequency offset between the two beams. This places an upper limit on the the rate of phase shift between the two input beams (i.e. the speed of an object or surface being measured) which can be tracked by heterodyne interferometry. For light at 632 nm with a frequency bias of 50 MHz, this limit is 30 meters/second with practical values of 10 m/s being more typical.
Also used as a means of circumventing the drawbacks of the canonical interferometer is a homodyne interferometer which uses orthogonally polarized test beams to produce two sets of output signals modulated quadrature with each other as shown in FIG. 3. This may be done using free-space or fiber optics as described in U.S. Pat. No. 5,663,793 issued to de Groot and U.S. Pat. No. 5,323,258 issued to Tsushima et al. In this way, when one pair of output signals is at their sensitivity null, the second pair of signals is at their sensitivity maximum. Aggregate sensitivity is therefore prevented from dropping to zero over the full period of input phase shifts. Also, the fact that output signals appear in quadrature allows the phase reversal ambiguity to be overcome. When one signal pair is at an ambiguity point (as illustrated in FIG. 3), the quadrature signals have clear slopes which indicate the direction of the phase shift advance.
Polarization sensitive homodyne interferometers also have drawbacks. One such drawback is again the increased complexity of the device due to the use of additional polarization beamsplitters and wave plates. Furthermore, great care must be used to prevent stray backreflections from the surfaces of these many components. Such backreflections produce interferometric noise which degrades the measurement accuracy. Also, care must be exercised to prevent birefringence in the system which could cross-couple the two independent polarizations and confuse interpretation of the detected signals.
Yet another method which bypasses the problems of the canonical interferometer is the multiphase approach as described in U.S. Pat. No. 5,392,116 issued to Makosch. In this method, orthogonally polarized beams are again created and used to carry the sought-after phase information (which was imparted to the beams by height differences between two points on a sample surface). However, instead of creating just two pairs of interfering beams using separate beamsplitters, a diffraction grating is used to create 5 pairs of beams via the 0th, xc2x11st and xc2x12nd diffraction orders. A special phase shifter element is used to impart predetermined phase delays onto the orthogonally polarized components of the diffracted orders so that when they are interfered, five sets of quadrature signals are produced. It is claimed in the ""116 patent that the multiplicity of quadrature signals improves the accuracy of the measurement. It is also claimed that this method is preferred over the three phase, time-sequenced measurement technique described in U.S. Pat. No. 4,298,283 issued to Makosch et al.
While the Makosch approach represents a more compact and accurate implementation of homodyne quadrature interferometry, it is still relatively complicated and expensive to construct. In particular, great care must be exercised in fabricating the special phase shifting element. Care is also needed when aligning the multiple beams to produce overlap for suitable interference to take place.
U.S. Pat. No. 5,875,029 issued to Jann et al. discloses an interferometric surface inspection apparatus that reflects a reference light beam from an inclined reference surface and an object light beam from a disk surface. The beams are then interfered to create a multifringe spatial interference pattern. This pattern is split by a prism and directed towards three separate photodetectors. In front of each photodetector is a mask pattern, referred to as a xe2x80x9crulingxe2x80x9d, which either blocks or transmits the light of the interference pattern depending on the spatial location of the fringes of the interference pattern. By orienting the positions of the masks with respect to the interference fringes, the signals at the detectors can be brought into a quadrature relation which can give the interferometer null-free sensitivity. The signals can then be used to compute variations in the height of a storage disk surface.
As follows from the limitations of all these previous approaches, there is still a need in the art for a simple, compact and relatively inexpensive interferometric measuring technique that is accurate, has no sensitivity dropouts, or ambiguity points, is easy to construct and free of critical alignment requirements.
Of relevance to the present invention is U.S. Pat. No. 4,732,447 issued to Wright et al. Disclosed in the ""447 patent is a homodyne optical coherent receiver for communication systems which is phase insensitive. The receiver is based on a multiport optical coupler., and in the case of 3 ports, uses the 120 degree phase separation between the light beams in the 3 -port coupler to effect a more robust receiver. The ""447 patent however does not discuss the advantage of the 3 -port coupler as a means for accurately determining the phase-difference between the input beams or suggest its use for metrological applications.
Also of relevance to the present invention is U.S. Pat. No. 5,313,266 issued to Keolian et. al. The ""266 patent discloses an interferometer sensor comprising, among other things, a first coupler for splitting a source into two beams, a second coupler which produces three phase modulated signals x, y, and z, and a symmetric demodulator circuit for converting the set of three electrical signals a, b, and c into a single electrical signal which is proportional to the signal of interest. The application specified in the patent covers hydrophone signal demodulation, and the emphasis of the patent is on the method of symmetric demodulation as accomplished by the algorithm and analog circuitry described therein.
The ""266 patent specifically references articles in the scientific literature which describe the important characteristics of the 3xc3x973 coupler which makes it useful for interferometric applications. In these articles, the specific application discussed was that of a fiber gyroscope. These articles includes Sheem, xe2x80x9cFiberoptic Gyroscopes with 3xc3x973 Directional Couplerxe2x80x9d, Appl. Phys. Lett., 37(10), pp. 869-871, Nov. 15, 1980; and, Sheem, xe2x80x9cOptical Fiber Interferometers with 3xc3x973 Directional Couplersxe2x80x9d, in J. Appl. Phys., 52 (6), pp.3865-3872, June 1981.
One embodiment of the present invention is an interferometer that contains a tri-coupler to mix light from three different waveguides. The light is emitted from a light source and may be reflected from a test surface. The output of the tri-coupler may be three different light beams that have intensities 120 degrees out of phase from each other. The out of phase light beams may be detected by a plurality of photodetectors. The detected out of phase light beams may be used to determine a height of the test surface.